In a printing process where a printhead has multiple nozzles, not every nozzle reacts to a standard drive waveform the same way, i.e., each nozzle can produce a droplet of slightly different volume. In situations where the nozzles are relied upon to deposit fluid droplets into respective fluid deposition areas (“target regions”), lack of consistency can lead to problems. This is particularly the case for manufacturing applications, where the ink transports a material that will become a permanent thin-film structure within an electronic device. One example application where this issue arises is in a manufacturing process applied to the fabrication of displays, such as organic light-emitting diode (“OLED”) displays, as used for small and large electronic devices (e.g., for portable devices, large scale high-definition television panels and other devices). Where a printing process is used to deposit an ink carrying light-generating materials of such displays, the volume discrepancy across rows or columns of pixels contributes to visible lighting or color defects in a displayed image. Note that “ink” as used herein refers to any fluid applied to a substrate by nozzles of a printhead irrespective of color characteristics; for example, in the mentioned OLED display fabrication application, ink is typically deposited in place and then processed, dried or cured in order to directly form a permanent material layer, and this process might be repeated with the same ink or a different ink to form several such layers.
FIG. 1A is used to introduce this nozzle-droplet inconsistency issue, with an illustrative diagram generally referenced using numeral 101. In FIG. 1A, a printhead 103 is seen to have five ink nozzles, which are each depicted using small triangles at the bottom of the printhead, each respectively numbered (1)-(5). Note that in a typical manufacturing application, there can be many more than five nozzles, e.g., 24-10,000, depending on application; in the case of FIG. 1A, five nozzles are referenced simply for ease of understanding. It should be assumed that in an example application, it is desired to deposit fifty picoliters (50.00 pL) of a fluid into each of five specific target regions of an array of such regions, and further, that each of five nozzles of a printhead is supposed to eject ten picoliters (10.00 pL) of fluid with each relative movement (“pass” or “scan”) between the printhead and a substrate into each of the various target regions. The target regions can be any surface areas of the substrate, including adjoining unseparated areas (e.g., such that deposited fluid ink partially spreads to blend together between regions), or respective, fluidically-isolated regions. These regions are generally represented in FIG. 1A using ovals 104-108, respectively. Thus, it might be assumed that exactly five passes of the printhead are necessary as depicted to fill each of the five specific target regions. However, printhead nozzles will in practice have some minor variations in structure or actuation, such that a given drive waveform applied to respective nozzle transducers yields slightly different droplet volumes for each nozzle. As depicted in FIG. 1A, for example, the firing of nozzle (1) yields a droplet volume of 9.80 picoliters (pL) with each pass, with five 9.80 pL droplets being depicted within oval 104. Note that each of the droplets is represented in the figure by a distinct location within the target region 104, but in practice, the location of each of the droplets may be the same or may overlap. Nozzles (2)-(5), by contrast, yield slightly different, respective droplet volumes of 10.01 pL, 9.89 pL, 9.96 pL and 10.03 pL. With five passes between printhead and substrate where each nozzle deposits fluid on a mutually-exclusive basis into the target regions 104-108, this deposition would result in a total deposited ink volume variation of 1.15 pL across the five target regions; this can be unacceptable for many applications. For example, in some applications, discrepancy of as little as one percent (or even much less) in deposited fluid can cause issues; in the case of OLED display fabrication, such variation can potentially result in image artifacts observable in a finished display.
Manufacturers of televisions and other forms of displays will therefore effectively specify precise volume ranges that must be observed with a high-degree of precision, e.g., 50.00 pL, ±0.25 pL in order for a resultant product to be considered acceptable; note that in this exemplary case, the specified tolerance must be within one-half percent of the target of 50.00 pL. In an application where each nozzle represented by FIG. 1A was to deposit into pixels in respective horizontal lines of a high-definition television (“HDTV”) screen, the depicted variation of 49.02 pL-50.17 pL might therefore yield unacceptable quantity, because this would represent about a ±1.2% variation (e.g., instead of the desired maximum tolerance of ±0.5% variation). While display technologies have been cited as an example, it should be understood that the nozzle-droplet inconsistency problem can arise in other contexts.
In FIG. 1A, nozzles are specifically aligned with target regions (e.g., wells) such that specific nozzles print into specific target regions. In FIG. 1B, an alternate case 151 is shown in which the nozzles are not specially aligned, but in which nozzle density is high relative to target region density; in such a case, whichever nozzles happen to traverse specific target regions during a scan or pass are used to print into those target regions, with potentially several nozzles traversing each target region in each pass. In the example shown, the printhead 153 is seen to have five ink nozzles and the substrate is seen to have two target regions 154-155, each located such that nozzles (1) and (2) will traverse target region 154, nozzles (4) and (5) will traverse target region 155, and nozzle (3) will not traverse either target region. As shown, in each pass, one or two droplets are deposited into each well, as depicted. Note that once again, the droplets can be deposited in a manner that is overlapping or at discrete points within each target region, and that the particular illustration in FIG. 1B is illustrative only; as with the example presented in FIG. 1A, it is once again assumed that it is desired to deposit fifty picoliters (50.00 pL) of a fluid into each of target regions 154-155, and that each nozzle has a nominal droplet volume of approximately 10.00 pL. Utilizing the same per nozzle droplet volume variation as observed in connection with the example of FIG. 1A, and assuming that each nozzle that overlaps with a target region on a given pass will deliver a droplet into that target region up until a total of five droplets have been delivered, it is observed that the target regions are filled in three passes and there is a total deposited ink volume variation from the target of 50.00 pL of 0.58 pL across the two target regions, and further a discrepancy outside of specified tolerance; again, this can be unacceptable for many applications.
It is noted that in connection with the examples above, the droplet consistency issue is further exacerbated by the issue that droplet volumes can statistically vary, even for a given nozzle and given drive waveform. Thus, in the examples discussed above, it was assumed that nozzle (1) of the printhead from FIGS. 1A and 1B would produce a droplet volume of 9.80 pL in response to a given drive waveform but, in practice, in a real world case, droplet volume can be assumed to vary somewhat depending on various factors, for example, process, voltage, temperature, printhead age and many other factors, such that actual droplet volume may not be precisely known.
While techniques have been proposed to address the droplet consistency problem, generally speaking, these techniques either still do not reliably provide fill volumes that stay within the desired tolerance range or they dramatically increase manufacturing time and cost, i.e., they are inconsistent with a goal of having high quality with a low consumer price-point; such quality and low price-point can be key for applications where commodity products, such as HDTVs, are concerned.
What is therefore needed are techniques useful in depositing fluid into target regions of a substrate using a printhead with nozzles. More specifically, what is needed are techniques for precisely controlling deposited fluid volumes in respective target regions of a substrate notwithstanding variations in nozzle-droplet ejection volumes, ideally on a cost-effective basis that permits fast fluid deposition operations and thus improves the speed of device fabrication. The techniques described below satisfy these needs and provide further, related advantages.
The subject matter defined by the enumerated claims may be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings. This description of one or more particular embodiments, set out below to enable one to build and use various implementations of the technology set forth by the claims, is not intended to limit the enumerated claims, but to exemplify their application. Without limiting the foregoing, this disclosure provides several different examples of techniques used to fabricate a materials layer by planning printhead movement so as to maintain deposited ink volume within predetermined allowances while not excessively increasing the number of printhead passes (and thus the time needed to complete a deposited layer). In connection with these techniques, accurate droplet measurement can be performed so as to accurately plan composite ink fills in any target region, with measurement highly integrated with production printing. The various techniques can be embodied as software for performing these techniques, in the form of a computer, printer or other device running such software, in the form of control data (e.g., a print image) for forming a materials layer, as a deposition mechanism, or in the form of an electronic or other device (e.g., a flat panel device or other consumer end product) fabricated as a result of these techniques. While specific examples are presented, the principles described herein may also be applied to other methods, devices and systems as well.